By Richard Cowan

WASHINGTON (Reuters) - Washington is trying to gauge whether Russia's recent bomber mission near a U.S. aircraft carrier indicated Moscow's return to a Cold War "mind-set" and is considering how the Pentagon should respond, a senior U.S. military officer said on Tuesday.

But other senior U.S. defense and Navy officials stressed they did not see Russia's weekend bomber flights south of Japan as provocative.

Four U.S. fighter jets were scrambled on February 9 to escort Russian bombers that approached the USS Nimitz south of Japan. One Russian bomber flew over the deck of the aircraft carrier, escorted by a U.S. fighter jet.

Adm. Gary Roughead, U.S. chief of naval operations, downplayed the incident and said it reflected Russia's emerging naval power.

"I think what we are seeing is a Russian military or Russian navy that is emerging and, in the case of the navy, desiring to emerge as a global navy," Roughead told reporters at the Pentagon.

"I do not consider it to be provocative," he said of the bomber mission.

But on Capitol Hill, another top U.S. military officer -- Marine Corps Gen. James Cartwright -- said the Pentagon was trying to assess the implications of Russia's actions.

"Now, what we're concerned about is what are the indications of this return to a Cold War mind-set, what are the implications of that activity and how do we best address that," said Cartwright, vice chairman of the Joint Chiefs of Staff.

The incident happened in neutral international airspace, Cartwright said.

"We're just trying to go back and look at what message was intended by this overflight," he told a Senate panel.

At the State Department, spokesman Sean McCormack said the Russian bomber flights were not seen as a threat.

"The Russians made a decision to resume some of their long-range aviation flights, involving some of their assets left over from the Cold War," he told reporters.

"I don't think we view it as a particular threat. It is something that we watch closely, and I'm sure folks over at the Pentagon watch it as well."

Any U.S. expressions of concern to Russia would probably be carried out through military channels, McCormack said.

TESTY RELATIONS

U.S.-Russian relations have become testy, with Washington concerned that Russian democracy is being eroded and Moscow complaining of U.S. interference.

A dispute over U.S. plans to place missile defense assets in former Soviet-allied territory has also raised tensions, and Russia is unhappy with continued U.S. support for expansion of the NATO military alliance.

Russian officials have said they will revive some of the military power and reach that was allowed to collapse with the Soviet Union.

Russia could train its nuclear missiles on Ukraine if the pro-Western state joins NATO, Russian President Vladimir Putin said in Moscow on Tuesday.

Asked his reaction to Putin's statement, McCormack said: "There he goes again." McCormack offered no further comment, saying he had not seen Putin's remarks.

The Russian Air Force said the mission by four Tu-95 bombers was part of long-distance patrols in the Pacific, Atlantic and Arctic oceans and the Black Sea that began last August.

A Russian news agency quoted Air Force spokesman Alexander Drobyshevsky expressing surprise at "all the clamor this raised."

The last time a Russian bomber flew over a U.S. aircraft carrier was in July 2004, and Russian bombers have increased their flights near U.S. territory to demonstrate their long-range strike capability.

Sen. Bill Nelson, a Florida Democrat who raised the issue during the Senate hearing, said the Russian maneuver "sounds pretty provocative to me." He said the Armed Services Committee, of which he is a member, would look into the issue.

(Additional reporting by Dmitry Solovyov in Moscow)

(Reporting by Richard Cowan and Kristin Roberts, Editing by John O'Callaghan)